1. Field of the Invention
The invention relates to a bone screw, especially to a bone screw of minimally invasive fixation device for lumbar.
2. Description of the Related Art
Any displacement of or compression on the spine or even the tension of surrounding muscles or ligaments can directly influence the nerves in the spine and indirectly lead to difficulty with moving, causing aches, or discomfort caused by the organs located at the distal ends which the nerves reach, the muscles, or glands. Generally, a surgery to install a fixation device between the vertebras can be applied to treat such symptoms for releasing the pressure. In some cases, an intervertebral discectomy is performed to remove the malfunctioning intervertebral discs between the vertebras and an interbody fusion cage is arranged at the removing position for reconstructing the height of adjacent vertebras, and fixation device is installed between the vertebras. As a result, the vertebras can be secured and aligned in the following healing procedures.
In the conventional surgery of installing a fixation device, bone screws are first implanted into and secured to the upper and lower two vertebras or the appropriate positions of multiple vertebras to be secured. The bone screws have rod securing bases at each top end for securing a rod between the bone screws to connect and configure the bone screws into a rigid securing rod structure so that the positioning between the vertebras is maintained and an appropriate supporting force is provided to achieve the function of releasing pressure and maintaining alignment.
Up to now, minimally invasive surgeries of spinal fixation device have been applied in treating pathological changes of vertebras since such surgeries result in small wounds, largely reduce damage to the treating portions and the neighboring tissues for improving the safety of operation, and decrease the time period of healing and recovery.
A conventional technique such as US publication No. US 2012/0323278 A1, entitled “MINIMALLY INVASIVE SPINAL STABILIZATION SYSTEM,” discloses a minimally invasive spinal fixation device for providing relative fixation of a series of vertebras comprising a securing rod and a plurality of bone screws. Each bone screw includes an upper portion and a thread body. The upper portion is configured to receive a portion of the securing rod. The thread body extends from a first end of the upper portion and is configured to engage one of the vertebras. The device further comprises an engaging member which includes a plurality of outer threads configured to engage a plurality of threads correspondingly formed on an inner surface of the upper portion. The securing rod is secured to the upper portion by the engaging member. The upper portion comprises a separation area which is weaker than the other portions of the upper portion so that a position is defined for easily separating the first portion of the upper portion from the other portions of the upper portion at the position when a sufficient force is applied to at least the first portion of the upper portion.
The specific structure of the bone screw is illustrated in FIG. 4. A bone screw has an upper portion 30 and a thread body 40. The upper portion 30 of the bone screw is in the form of a tube having a U-shaped hollow portion 35. An annular notch 36 in the upper portion divides the upper portion 30 into two portions: a securing base 31 and a separation area 32. While operating the minimally invasive surgery of the spinal fixation device, the thread bodies 40 of multiple bone screws are respectively screwed into multiple vertebras, but the opening at the top end of the upper portion 30 remains above the skin. A ruler-like alignment fixture is applied to align the upper portion 30 of each bone screw for aligning the height of the implanted bone screws and making the longitudinal axes of the U-shaped hollow portions 35 of the bone screw upper portions 30 parallel with each other so that a securing rod can be arranged on the securing bases 31 of the bone screw upper portions. Then, the opening at the top of the upper portion 30 is covered with a cap 60 for supporting and stabilizing the separation area 32 which is a relatively weaker structure at the upper two sides of the upper portion 30. As a result, the successive installation of the securing rod and removing of the separation area 32 at the two sides of the upper portions 30 go well and smoothly.
The upper portion 30 of the bone screw is split at its upper portion to form the separation area 32, so the structure of the separation area 32 is relatively weak. While encountering an external force under operation of implanting bone screws or fixing the securing rod, the bone screw could break or deform to impede the surgery and even adversely influence the result of the surgery. Even though a cap 60 is installed at the top opening of the upper portion to address the aforementioned shortcoming, such complicated structure not only increases the complexity of assembling these components during surgery but also increases the time period and risk of the surgery. Moreover, although the conventional structure of a bone screw allows a small portion of the upper portion 30 to remain above the skin for inserting a ruler-like alignment fixture after implantation of the bone screw, owing to the difference in angle of each vertebra, it is still not easy to accurately conform the height of the implanted bone screws to the curvature of the spine with the ruler-like alignment fixture. In addition, the ruler-like alignment fixture is to align the positions and angles of the upper portions 30 of multiple bone screws by receiving the small top portions of the upper portions in a guiding slot. However, the cross-section of the U-shaped hollow portion 35 of the upper portion 30 is perpendicular to the axis of the guiding slot of the alignment fixture, so it is difficult to make the longitudinal axes of the U-shaped hollow portion 35 of the upper portion 30 accurately parallel to each other through the alignment by the guiding slot. Failing to make the longitudinal axes of the U-shaped hollow portion 35 parallel to each other will lead to interference while installing the securing rod, and thus installing the securing rod on the securing bases 31 of the bone screw upper portions cannot be smoothly accomplished.